Species of corn rootworm are considered to be the most destructive corn pests. In the United States the three important species are Diabrotica virgifera virgifera, the western corn rootworm; D. longicornis barberi, the northern corn rootworm and D. undecimpunctata howardi, the southern corn rootworm. Only western and northern corn rootworms are considered primary pests of corn in the US Corn Belt. Corn rootworm larvae cause the most substantial plant damage by feeding almost exclusively on corn roots. This injury has been shown to increase plant lodging, to reduce grain yield and vegetative yield as well as alter the nutrient content of the grain. Larval feeding also causes indirect effects on maize by opening avenues through the roots for bacterial and fungal infections which lead to root and stalk rot diseases. Adult corn rootworms are active in cornfields in late summer where they feed on ears, silks and pollen, interfering with normal pollination.
Corn rootworms are mainly controlled by intensive applications of chemical pesticides, which are active through inhibition of insect growth, prevention of insect feeding or reproduction, or cause death. Good corn rootworm control can thus be reached, but these chemicals can sometimes also affect other, beneficial organisms. Another problem resulting from the wide use of chemical pesticides is the appearance of resistant insect varieties. Yet another problem is due to the fact that corn rootworm larvae feed underground thus making it difficult to apply rescue treatments of insecticides. Therefore, most insecticide applications are made prophylactically at the time of planting. This practice results in a large environmental burden. This has been partially alleviated by various farm management practices, but there is an increasing need for alternative pest control mechanisms.
Biological pest control agents, such as Bacillus thuringiensis (Bt) strains expressing pesticidal toxins like δ-endotoxins, have also been applied to crop plants with satisfactory results against primarily lepidopteran insect pests. The δ-endotoxins are proteins held within a crystalline matrix that are known to possess insecticidal activity when ingested by certain insects. The various δ-endotoxins have been classified based upon their spectrum of activity and sequence homology. Prior to 1990, the major classes were defined by their spectrum of activity with the Cry1 proteins active against Lepidoptera (moths and butterflies), Cry2 proteins active against both Lepidoptera and Diptera (flies and mosquitoes), Cry3 proteins active against Coleoptera (beetles) and Cry4 proteins active against Diptera (Hofte and Whitely, 1989, Microbiol. Rev. 53:242–255). Recently a new nomenclature was developed which systematically classifies the Cry proteins based on amino acid sequence homology rather than insect target specificities (Crickmore et al. 1998, Microbiol. Molec. Biol. Rev. 62:807–813).
The spectrum of insecticidal activity of an individual δ-endotoxin from Bt is quite narrow, with a given δ-endotoxin being active against only a few species within an Order. For instance, the Cry3A protein is known to be very toxic to the Colorado potato beetle, Leptinotarsa decemlineata, but has very little or no toxicity to related beetles in the genus Diabrotica (Johnson et al., 1993, J. Econ. Entomol. 86:330–333). According to Slaney et al. (1992, Insect Biochem. Molec. Biol. 22:9–18) the Cry3A protein is at least 2000 times less toxic to southern corn rootworm larvae than to the Colorado potato beetle. It is also known that Cry3A has little or no toxicity to the western corn rootworm.
Specificity of the δ-endotoxins is the result of the efficiency of the various steps involved in producing an active toxin protein and its subsequent interaction with the epithelial cells in the insect mid-gut. To be insecticidal, most known δ-endotoxins must first be ingested by the insect and proteolytically activated to form an active toxin. Activation of the insecticidal crystal proteins is a multi-step process. After ingestion, the crystals must first be solubilized in the insect gut. Once solubilized, the δ-endotoxins are activated by specific proteolytic cleavages. The proteases in the insect gut can play a role in specificity by determining where the δ-endotoxin is processed. Once the δ-endotoxin has been solubilized and processed it binds to specific receptors on the surface of the insects' mid-gut epithelium and subsequently integrates into the lipid bilayer of the brush border membrane. Ion channels then form disrupting the normal function of the midgut eventually leading to the death of the insect.
In Lepidoptera, gut proteases process δ-endotoxins from 130–140 kDa protoxins to toxic proteins of approximately 60–70 kDa. Processing of the protoxin to toxin has been reported to proceed by removal of both N- and C-terminal amino acids with the exact location of processing being dependent on the specific insect gut fluids involved (Ogiwara et al., 1992, J. Invert. Pathol. 60:121–126). The proteolytic activation of a 6-endotoxin can play a significant role in determining its specificity. For example, a δ-endotoxin from Bt var. aizawa, called IC1, has been classified as a Cry1Ab protein based on its sequence homology with other known Cry1Ab proteins. Cry1Ab proteins are typically active against lepidopteran insects. However, the IC1 protein has activity against both lepidopteran and dipteran insects depending upon how the protein is processed (Haider et al. 1986, Euro. J. Biochem. 156: 531–540). In a dipteran gut, a 53 kDa active IC1 toxin is obtained, whereas in a lepidopteran gut, a 55 kDa active IC1 toxin is obtained. IC1 differs from the holotype HD-1 Cry1Ab protein by only four amino acids, so gross changes in the receptor binding region do not seem to account for the differences in activity. The different proteolytic cleavages in the two different insect guts possibly allow the activated molecules to fold differently thus exposing different regions capable of binding different receptors. The specificity therefore, appears to reside with the gut proteases of the different insects.
Coleopteran insects have guts that are more neutral to acidic and coleopteran-specific δ-endotoxins are similar to the size of the activated lepidopteran-specific toxins. Therefore, the processing of coleopteran-specific δ-endotoxins was formerly considered unnecessary for toxicity. However, recent data suggests that coleopteran-active δ-endotoxins are solubilized and proteolyzed to smaller toxic polypeptides. The 73 kDa Cry3A δ-endotoxin protein produced by B. thuringiensis var. tenebrionis is readily processed in the bacterium at the N-terminus, losing 49–57 residues during or after crystal formation to produce the commonly isolated 67 kDa form (Carroll et al., 1989, Biochem. J. 261:99–105). McPherson et al., 1988 (Biotechnology 6:61–66) also demonstrated that the native cry3A gene contains two functional translational initiation codons in the same reading frame, one coding for the 73 kDa protein and the other coding for the 67 kDa protein starting at Met-1 and Met-48 respectively, of the deduced amino acid sequence (See SEQ ID NO: 2). Both proteins then can be considered naturally occurring full-length Cry3A proteins. Treatment of soluble 67 kDa Cry3A protein with either trypsin or insect gut extract results in a cleavage product of 55 kDa with Asn-159 of the deduced amino acid sequence at the N-terminus. This polypeptide was found to be as toxic to a susceptible coleopteran insect as the native 67 kDa Cry3A toxin. (Carroll et al. Ibid). Thus, a natural trypsin recognition site exists between Arg-158 and Asn-159 of the deduced amino acid sequence of the native Cry3A toxin (SEQ ID NO: 2). Cry3A can also be cleaved by chymotrypsin, resulting in three polypeptides of 49, 11, and 6 kDa. N-terminal analysis of the 49 and 6 kDa components showed the first amino acid residue to be Ser-162 and Tyr-588, respectively (Carroll et al., 1997 J. Invert. Biol. 70:41–49). Thus, natural chymotrypsin recognition sites exist in Cry3A between His-161 and Ser-162 and between Tyr-587 and Tyr-588 of the deduced amino acid sequence (SEQ ID NO: 2). The 49 kDa chymotrypsin product appears to be more soluble at neutral pH than the native 67 kDa protein or the 55 kDa trypsin product and retains full insecticidal activity against the Cry3A-susceptible insects, Colorado potato beetle and mustard beetle, (Phaedon cochleariae).
Insect gut proteases typically function in aiding the insect in obtaining needed amino acids from dietary protein. The best understood insect digestive proteases are serine proteases that appear to be the most common (Englemann and Geraerts, 1980, J. Insect Physiol. 261:703–710), particularly in lepidopteran species. The majority of coleopteran larvae and adults, for example Colorado potato beetle, have slightly acidic midguts, and cysteine proteases provide the major proteolytic activity (Wolfson and Mudock, 1990, J. Chem. Ecol. 16:1089–1102). More precisely, Thie and Houseman (1990, Insect Biochem. 20:313–318) identified and characterized the cysteine proteases, cathepsin B and H, and the aspartyl protease, cathepsin D in Colorado potato beetle. Gillikin et al. (1992, Arch. Insect Biochem. Physiol. 19:285–298) characterized the proteolytic activity in the guts of western corn rootworm larvae and found 15, primarily cysteine, proteases. Until disclosed in this invention, no reports have indicated that the serine protease, cathepsin G, exists in western corn rootworm. The diversity and different activity levels of the insect gut proteases may influence an insect's sensitivity to a particular Bt toxin.
Many new and novel Bt strains and δ-endotoxins with improved or novel biological activities have been described over the past five years including strains active against nematodes (EP 0517367A1). However, relatively few of these strains and toxins have activity against coleopteran insects. Further, none of the now known coleopteran-active δ-endotoxins, for example Cry3A, Cry3B, Cry3C, Cry7A, Cry8A, Cry8B, and Cry8C, have sufficient oral toxicity against corn rootworm to provide adequate field control if delivered, for example, through microbes or transgenic plants. Therefore, other approaches for producing novel toxins active against corn rootworm need to be explored.
As more knowledge has been gained as to how the δ-endotoxins function, attempts to engineer δ-endotoxins to have new activities have increased. Engineering δ-endotoxins was made more possible by the solving of the three dimensional structure of Cry3A in 1991 (Li et al., 1991, Nature 353:815–821). The protein has three structural domains: the N-terminal domain I, from residues 1–290, consists of 7 alpha helices, domain II, from residues 291–500, contains three beta-sheets and the C-terminal domain III, from residues 501–644, is a beta-sandwich. Based on this structure, a hypothesis has been formulated regarding the structure/function relationship of the δ-endotoxins. It is generally thought that domain I is primarily responsible for pore formation in the insect gut membrane (Gazit and Shai, 1993, Appl. Environ. Microbiol. 57:2816–2820), domain II is primarily responsible for interaction with the gut receptor (Ge et al., 1991, J. Biol. Chem. 32:3429–3436) and that domain III is most likely involved with protein stability (Li et al. 1991, supra) as well as having a regulatory impact on ion channel activity (Chen et al., 1993, PNAS 90:9041–9045).
Lepidopteran-active δ-endotoxins have been engineered in attempts to improve specific activity or to broaden the spectrum of insecticidal activity. For example, the silk moth (Bombyx mori) specificity domain from Cry1Aa was moved to Cry1Ac, thus imparting a new insecticidal activity to the resulting chimeric protein (Ge et al. 1989, PNAS 86: 4037–4041). Also, Bosch et al. 1998 (U.S. Pat. No. 5,736,131), created a new lepidopteran-active toxin by substituting domain III of Cry1E with domain III of Cry1C thus producing a Cry1E–Cry1C hybrid toxin with a broader spectrum of lepidopteran activity.
Several attempts at engineering the coleopteran-active δ-endotoxins have been reported. Van Rie et al., 1997, (U.S. Pat. No. 5,659,123) engineered Cry3A by randomly replacing amino acids, thought to be important in solvent accessibility, in domain II with the amino acid alanine. Several of these random replacements confined to receptor binding domain II were reportedly involved in increased western corn rootworm toxicity. However, others have shown that some alanine replacements in domain II of Cry3A result in disruption of receptor binding or structural instability (Wu and Dean, 1996, J. Mol. Biol. 255: 628–640). English et al., 1999, (Intl. Pat. Appl. Publ. No. WO 99/31248) reported amino acid substitutions in Cry3Bb that caused increases in toxicity to southern and western corn rootworm. However, of the 35 reported Cry3Bb mutants, only three, with mutations primarily in domain II and the domain II-domain I interface, were active against western corn rootworm. Further, the differences in toxicity of wild-type Cry3Bb against western corn rootworm in the same assays were greater than any of the differences between the mutated Cry3Bb toxins and the wild-type Cry3Bb. Therefore, improvements in toxicity of the Cry3Bb mutants appear to be confined primarily to southern corn rootworm.
There remains a need to design new and effective pest control agents that provide an economic benefit to farmers and that are environmentally acceptable. Particularly needed are modified Cry3A toxins that control western corn rootworm, the major pest of corn in the United States, that are or could become resistant to existing insect control agents. Furthermore, agents whose application minimizes the burden on the environment, as through transgenic plants, are desirable.